1. Field of the Invention
The present invention relates to an ink jet recording apparatus and an ink jet recording method capable of recording the half-tone image in such a manner as to change the number of recording dots per unit area, as well as recorded matters.
2. Related Background Art
In the conventional ink jet recording methods, the recording is performed in such a manner as to discharge the ink through a plurality of discharge orifices formed in a recording head in accordance with data signal and attach ink droplets onto the recording medium such as a paper. This recording method has been utilized for printers, facsimile apparatuses, or copying machines, for example.
For the above apparatuses, there is a method involving the use of electricity-heat energy converters in which heat generating elements (electrothermal energy converters or electricity-heat energy converters) are provided in the neighborhood of discharge orifices to discharge the ink and apply an electrical signal to those heat generating elements to heat the ink locally, thus causing a pressure change therein to discharge the ink through discharge orifices, or the use of electromechanical converters such as piezo-electrical elements.
In this type of recording method, the half-tone recording is controlled in accordance with a dot density control method in which the half-tone is represented by controlling the number of recording dots per unit area with the recording dot of fixed size, or a dot diameter control method in which the half-tone is represented by controlling the size of recording dots.
Herein, the latter dot diameter control method has some restrictions because of its complex control required to minutely change the size of recording dot, and therefore the former dot density control method is generally employed.
When the electricity-heat energy converters are used as ink discharge means, they can be easily manufactured and allows for the high density, and thus the high resolution, but has the difficulty in controlling the amount of pressure change, so that the diameter of recording dot can not be readily modulated. Hence, the dot density control method is mainly employed for the half-tone recording with the ink jet recording method.
Typical of the binarization method for the half-tone representation for use with this dot density control method is an organizational dither method, but this method has a problem that the number of gradations is limited by the matrix size. That is, to increase the number of gradations requires to increase the matrix size, but there is a problem that if the matrix size is increased, one pixel of recording image which is constituted of one matrix is larger, thereby resulting in lower resolution. Also another typical binarization method is a conditioned decision type dither method such as an error diffusion method. This is a method in which the threshold is changed in consideration of peripheral pixels around the input pixel, whereas the above-mentioned organizational dither method is an independent decision type dither method in which the binarization is made using a threshold value irrespective of the input pixel. The conditioned decision type dither method represented by this error diffusion method has the advantage that there is good compatibility between gradation and resolution, and that when the original image is a printed image, there are quite less moire patterns produced in the recorded image, whereas it has the drawback that the graininess becomes conspicuous in the light part of image, degrading the evaluation of image quality. This problem was remarkable particularly in the recording apparatus having low recording density.
To make the above graininess inconspicuous, a recording method has been proposed in which two recording heads for discharging the inks which are thin and thick in the dye concentration respectively are provided for the recording. With this method, the portion from the light part of image to the half-tone part has recording dots formed by the thin ink in the dye concentration, while the portion from the half-tone part to the dark part has recording dots formed by the thick ink. The dots formed by the thin ink in the dye concentration are light in the image density, while the dots by the dark ink in the dye concentration is dark in the image density.
FIG. 23 is a constitutional view showing the essence of a conventional color ink jet recording apparatus of the serial print type employing the dark/light ink.
Kk is a recording head for discharging a color ink of dark black, Ku is a recording head for discharging a color ink of light black, Ck is a recording head for discharging a color ink of dark cyan, Cu is a recording head for discharging a color ink of light cyan, Mk is a recording head for discharging a color ink of dark magenta, Mu is a recording head for discharging a color ink of light magenta, Yk is a recording head for discharging a color ink of dark yellow, and Yu is a recording head for discharging a color ink of light yellow. Each of the recording heads is installed a predetermined distance apart on a carriage 241.
The ink is supplied to each recording head from an ink cartridge 248 corresponding to respective color. Also, the control signal to recording head is provided via a flexible cable 249.
A recording medium composed of paper or plastic thin plate is passed by a conveying roller (not shown) and carried therewith by paper exhausting rollers 242 to be fed in a direction of the arrow by the driving of a conveying motor not shown.
Carriage 241 is guided and supported by means of a guide shaft 243, and an encoder not shown.
Carriage 241 is caused to reciprocate along the guide shaft 243 by the driving of a carriage motor 245 via a drive belt 244.
The inside of an ink discharge orifice of the recording head or a liquid channel through which the ink flows is provided with a heat generating element (electricity-heat energy converter) for generating the heat energy for use in discharging the ink.
In accordance with the read timing of encoder, the above-mentioned heat generating elements are driven on the basis of a recording signal to discharge ink droplets onto the recording medium in the order of dark black, light black, dark cyan, light cyan, dark magenta, light magenta, dark yellow, and light yellow, thereby forming an image.
At a home position of carriage selected out of the recording area, a recovery unit 246 having a cap portion 247 is disposed to effect the recovery of ink discharge performance and maintain the stability of ink discharge.
In the case of a so-called pictorial image in which the output image is represented in gradation, the reproduction of image with reduced graininess can be effected by making the effective use of the dark/light ink.
On the other hand, it is often preferred to perform the recording only by the use of dark ink, in the case of an image not requiring any gradation representation such as a document, graphics or listing which is composed of characters and line drawing, or an image already expanded in binary form by the computer.
For the purposes of achieving the compactness and the low price of the apparatus, and performing the recording by using the dark/light ink, a method is provided in which a recording head is used having a plurality of discharge orifice arrays for discharging different inks onto the same discharge orifice formation face of the same recording head. In this case, there is a problem that though the apparatus is smaller, the array of discharge orifices is divided corresponding to used ink color and the number of discharge orifices for each ink color is reduced, whereby the recording width per scan is narrower and the recording speed is decreased. Accordingly, the apparatus with its principal usage found only on the recording by the use of such dark/light ink is unsuitable for the output of document, graphic and listing image.